1. Field of the invention
The present invention relates to hydraulic pressure control device for an automatic transmission for a vehicle such as an automobile, and more particularly, relates to a hydraulic pressure control device having a reverse stage engagement prevention system for preventing the automatic transmission being set to the reverse stage under a certain vehicle operating condition such as forward traveling at or above a certain speed.
2. Description of the Prior Art
In the art of transmission for a vehicle such as an automobile it has already been proposed to incorporate in the hydraulic pressure control device of the automatic transmission a reverse stage prevention system for the purpose of failsafe such as described above, so that, even if the driver mistakenly carries out a manual shift change operation toward the reverse stage while the vehicle is traveling forward, the automatic transmission is not shifted to the reverse stage.
Such an art is disclosed in Japanese Patent Laying-open Publication Sho 60-37446 (1985). In this prior art, the reverse stage prevention system has a reverse stage disabling valve of a hydraulically operated spool type for carrying out a switching over operation according to supply or draining of a signal hydraulic pressure thereto, so as selectively to prevent supply of hydraulic pressure to a friction engaging device for setting up the reverse stage and to connect said friction engaging device to a drain.
In such a reverse speed stage disable valve the spool element should normally be kept definitely at a reverse speed stage enabling shift position so as not to disturb normal setting up of the reverse stage by the shift of the manual shift valve to the reverse range. However, in the above-mentioned prior art, the spool element which was once shifted to a reverse stage disabling shift position opposite to the reverse stage enabling shift position by a control hydraulic pressure is returned to the reverse speed stage enabling shift position only by a spring force when said hydraulic pressure was released. In such a structure there is a danger that the spool element sticks to the reverse stage disabling shift position and the setting up of the reverse stage is hindered.
Further, in the co-pending patent application No. 07/302,254, now U.S. Pat. No. 4,920,825 the present inventors have proposed and claimed a hydraulic pressure control device for an automatic transmission in which the selective prevention of the reverse stage is accomplished by sharing a common solenoid valve with a control system for controlling engagement and disengagement of the lock-up clutch for the torque converter so that the same solenoid valve controls a first spool type hydraulic pressure control valve for switching over the engagement and disengagement of the lock-up clutch and a second spool type hydraulic pressure control valve for selectively preventing the setting up of the reverse stage. In such a hydraulic pressure control device the spool element of said second hydraulic pressure control valve for the reverse stage control would uselessly reciprocate when the solenoid valve is turned on and off for the control of the lock-up clutch if no means were provided to isolate the reverse stage prevention system from the solenoid valve when the vehicle is traveling with the manual shift valve set to a forward range.